Alternating current ("AC") is commonly used to provide power to machines and appliances for industrial, commercial, and residential customers. Overcurrent protection is required for most installations, and industry standards provide guidelines for wire sizing and fuse rating. See National Electrical Code, Article 240. Fuses are held in place by "fuse blocks." A typical fuse block comprises a flat insulating body that is suitable for direct mounting via the rear surface inside a motor control panel or other electrical enclosure. A pair of terminal lugs are positioned on opposite ends of the body, and each lug has a fuse clip physically and electrically connected thereto. A conductor is connected to each of the terminal lugs, and a cartridge-type fuse is inserted into the clips to complete the circuit.
While fuses are generally effective protection against overcurrent conditions, certain transient voltage surges can occur so fast that the fuse fails to protect the connected device. It is known that a metal oxide varistor ("MOV") has an excellent voltage limiting characteristic and a very fast response time. Therefore, protection devices have been built that utilize an MOV to provide voltage surge protection. See, for example, U.S. Pat. No. 3,693,053 (Anderson) and U.S. Pat. No. 4,547,827 (Shedd).
However, while Anderson and Shedd teach ways to use MOV's to provide overvoltage protection, they do not teach a way to conveniently retrofit existing fuse blocks to add overvoltage protection. Further, it would be desirable to provide such protection at low cost and with minimal disruption of equipment operation.